Subwavelength grating groove type waveguide micro-ring resonant structure refractive index sensor based on series coupling

Abstract

The on-chip refractive index sensor comprises a silicon-based substrate, a buried oxide layer, a silicon waveguide layer and an upper cladding layer, the buried oxide layer grows on a first surface of the silicon-based substrate, the silicon waveguide layer grows on one surface, far away from the silicon-based substrate, of the buried oxide layer, and the upper cladding layer grows on the other surface of the buried oxide layer. And the upper cladding covers one surface, far away from the silicon-based substrate, of the buried oxide layer and wraps the silicon waveguide layer. The silicon waveguide layer is formed by connecting and coupling a micro-ring group, an upper straight waveguide and a lower straight waveguide in series, the straight waveguides and the micro-ring group are of sub-wavelength grating groove type structures, the micro-ring group comprises two or more SWGS micro-rings, and the left end and the right end of the upper SWGS straight waveguide are sequentially connected with input strip-shaped waveguides, namely sub-wavelength grating groove waveguide transition waveguide structures, which are in mirror symmetry. Compared with a traditional refractive index sensor based on a single nanowire waveguide resonant ring structure, the scheme has the advantages of higher detection sensitivity, larger measurable refractive index range and the like.

Description

Technical Field

[0001] The invention belongs to the technical field of micro-nano optical sensing, and in particular is a refractive index sensor based on a sub-wavelength grating groove (SWGS) waveguide micro-ring resonant structure based on serial coupling. Background Art

[0002] Nowadays, people have widely recognized the importance of photonic integrated circuits built on silicon-on-insulator (SOI). This is because such photonic integrated circuits have two major characteristics: high refractive index contrast and compatibility with current CMOS processing technology. Microring resonators (MRRs) enable light to travel one or several times around a microring to provide a fairly long light-matter interaction distance, and are one of the important technical solutions for realizing refractive index sensing of substances such as solutions and gases. However, the refractive index contrast between the silicon core and the cladding of traditional microring resonators is high, most of the electric field is confined to the core of the waveguide, and the overlapping area between the evanescent field and the analyte is limited. Therefore, sensors based on various types of microring resonance structures have been designed, such as ultra-thin waveguide microrings, sub-wavelength grating microrings, photonic crystal waveguide microrings, etc.

[0003] Ultra-thin waveguide microrings use the method of reducing the thickness of the waveguide. In the TM polarization direction of the light source, the waveguide will obtain deeper evanescent field penetration, which can improve the sensitivity of the device to 150nm / RIU. However, in the TE polarization direction of the light source, this method is ineffective in improving the sensitivity of the device. Photonic crystal waveguide microrings can simultaneously realize the refractive index sensing measurement of air mode and dielectric mode, but the improvement of the sensitivity of photonic crystal waveguide microrings is also very limited, usually only 200nm / RIU sensitivity can be achieved. Subwavelength grating microrings are new microring resonators designed based on subwavelength grating waveguides. Their waveguides are composed of two materials with different refractive indices arranged periodically. Due to the periodic structural design of the waveguide, light can have more contact with the analyte. The sensitivity of the subwavelength grating microring refractive index sensor has been able to reach 400nm / RIU, and it has become a more favorable choice for further improving the microring refractive index sensor. However, the refractive index sensing scheme based on a single subwavelength grating microring still has problems such as small free spectrum range and low sensitivity, and it is difficult to adapt to application scenarios requiring high-performance sensing. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art. In order to further improve the refractive index sensitivity and measurable refractive index range of a microring resonant sensor, the present invention proposes a refractive index sensing scheme based on a serially coupled sub-wavelength grating groove waveguide microring resonator.

[0005] The technical solution for achieving the purpose of the present invention is: an on-chip refractive index sensor based on a series-coupled sub-wavelength grating groove waveguide micro-ring resonant structure, comprising a silicon-based substrate, a buried oxide layer, a silicon waveguide layer and an upper cladding layer, wherein the buried oxide layer is grown on a first surface of the silicon-based substrate, the silicon waveguide layer is grown on a side of the buried oxide layer away from the silicon-based substrate, and the upper cladding layer covers a side of the buried oxide layer away from the silicon-based substrate and wraps the silicon waveguide layer.

[0006] Preferably, the silicon waveguide layer includes a microring group, a first straight waveguide and a second straight waveguide, the first straight waveguide and the second straight waveguide are arranged in parallel, the microring group is arranged between the first straight waveguide and the second straight waveguide, and both ends of the first straight waveguide are respectively connected with transition structures for realizing the transition between the input strip waveguide and the first straight waveguide or the second straight waveguide, and the transition structure is mirror-symmetrical with respect to the middle microring group, the first straight waveguide and the second straight waveguide.

[0007] Preferably, the microring group comprises at least two SWGS microrings.

[0008] Preferably, the radii of the SWGS microrings in the microring group are not exactly the same.

[0009] Preferably, each SWGS micro-ring comprises two SWG micro-rings with different radii, and the two SWG micro-rings form a concentric ring structure.

[0010] Preferably, the micro-ring group, the first straight waveguide and the second straight waveguide are all sub-wavelength grating groove structures.

[0011] Preferably, the first straight waveguide and the second straight waveguide and each SWGS microring in the microring group have the same number of silicon pillars.

[0012] Preferably, the sub-wavelength grating period of the first straight waveguide, the second straight waveguide and the micro-ring group satisfies λ / Λ>>2n eff , λ is the incident wavelength, Λ is the period length of the sub-wavelength grating, n eff is the effective refractive index of the waveguide.

[0013] Preferably, the coupling spacing between the microring group and the first straight waveguide and the second straight waveguide, the coupling spacing between the SWGS microrings in the microring group, and the gap spacing between the first straight waveguide, the second straight waveguide and the SWGS microring are less than 300 nm.

[0014] Preferably, the upper cladding material is air.

[0015] Compared with the prior art, the present invention has the following significant advantages:

[0016] 1. The present invention uses a single-groove or multi-groove sub-wavelength grating groove structure in the sensor. The light field in the structure is more concentrated in the gap area, which can further enhance the interaction between light and the substance to be measured, which is beneficial to improving the sensing sensitivity of the device.

[0017] 2. The present invention utilizes the vernier effect and adopts a series coupling of two or more SWGS microring resonant structure designs with different radii (the SWGS microring may include two or more SWG microrings with different radii). By ensuring that the resonance peaks of the transmission spectra of the respective SWGS microrings are aligned, a series SWGS microring refractive index sensor with a large FSR is obtained, which can increase the measurable refractive index range of the material to be measured, thereby expanding the sensing application scenarios.

[0018] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the structures particularly pointed out in the written description, claims, and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0020] Figure 1 This is a planar structural diagram of a sensor having two SWGS double-ring resonant structures with radii of 5 μm and 7.35 μm coupled in series in Example 1 of the present invention.

[0021] Figure 2 This is the cross-sectional structure of the upper cladding layer and the silicon waveguide layer on the SOI platform in Example 1 of the present invention.

[0022] Figure 3 1 and 2 are transmission spectra of the SWGS double-ring resonant structures with radii of 5 μm and 7.35 μm in pure water, respectively, in Example 1 of the present invention.

[0023] Figure 4 : This is the transmission spectrum of the sensor in pure water of two SWGS double-ring resonant structures with radii of 5 μm and 7.35 μm coupled in series in Example 1 of the present invention.

[0024] Figure 5 This is a curve showing the change of the resonant wavelength of the sensor with two SWGS double-ring resonant structures with radii of 5 μm and 7.35 μm coupled in series in Example 1 of the present invention as a function of the refractive index.

[0025] Figure 6This is the light field transmission distribution of the sensor of TE polarization in Example 1 of the present invention, in which two SWGS double-ring resonant structures with radii of 5 μm and 7.35 μm are coupled in series when the resonant wavelength is 1.57 μm.

[0026] Figure 7 This is a planar structural diagram of a sensor having two SWGS three-ring resonant structures with radii of 6.6 μm and 9.9 μm coupled in series in Example 1 of the present invention.

[0027] Figure 8 This is the transmission spectrum of the sensor in pure water of two SWGS three-ring resonant structures with radii of 6.6 μm and 9.9 μm coupled in series in Example 1 of the present invention.

[0028] Fig. 9 This is a curve showing the change of the resonant wavelength of the sensor with two SWGS three-ring resonant structures with radii of 6.6 μm and 9.9 μm coupled in series in Example 1 of the present invention as a function of the refractive index.

[0029] Reference numerals: silicon-based substrate-1, buried oxide layer-2, silicon waveguide layer-3 and upper cladding layer-4, SWGS double rings-5, 6, upper SWGS straight waveguide-7, lower SWGS straight waveguide-8, sub-wavelength grating waveguide-9, MMI converter-10, SWGS triple rings-11, 12. DETAILED DESCRIPTION

[0030] It is easy to understand that according to the technical solution of the present invention, without changing the essential spirit of the present invention, a person skilled in the art can imagine various embodiments of the present invention. Therefore, the following specific embodiments and drawings are only exemplary descriptions of the technical solution of the present invention, and should not be regarded as the whole of the present invention or as a limitation or limitation of the technical solution of the present invention. On the contrary, the purpose of providing these embodiments is to enable those skilled in the art to understand the present invention more thoroughly. The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the innovative concept of the present invention.

[0031] The present invention is conceived as follows: Figure 1 , 2 , 7, an on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure, comprising:

[0032] A silicon-based substrate 1, a buried oxide layer 2, a silicon waveguide layer 3 and an upper cladding layer 4, wherein the buried oxide layer 2 is grown on the first surface of the silicon-based substrate 1, the silicon waveguide layer 3 is grown on a side of the buried oxide layer 2 away from the silicon-based substrate 1, and the upper cladding layer 4 covers a side of the buried oxide layer 2 away from the silicon-based substrate 1 and wraps the silicon waveguide layer 3.

[0033] The silicon waveguide layer 3 is formed by a micro-ring group, a first straight waveguide 7 and a second straight waveguide 8 coupled in series. Both the straight waveguide and the micro-ring group are sub-wavelength grating groove structures. The micro-ring group includes two or more SWGS micro-rings. The multiple SWGS micro-rings in the micro-ring group are connected in series and coupled. The SWGS micro-ring is coupled with its adjacent first straight waveguide 7 or second straight waveguide 8. The left and right ends of the upper SWGS straight waveguide are sequentially connected with a mirror-symmetrical input strip waveguide-sub-wavelength grating groove waveguide transition waveguide structure.

[0034] In some embodiments, the first straight waveguide 7 and the second straight waveguide 8 are an upper SWGS straight waveguide and a lower SWGS straight waveguide, respectively.

[0035] In a further embodiment, the microring group comprises two upper and lower SWGS microrings, each of which comprises two SWG microrings of different radii, namely, SWGS double rings 5 ​​and 6. The radii of the two SWGS double rings are 5 μm and 7.35 μm, respectively.

[0036] In a further embodiment, the microring group includes two upper and lower SWGS microrings, each of which includes three SWG microrings of different radii, namely, SWGS three rings 11 and 12. The radii of the two SWGS microrings are 6.6 μm and 9.9 μm respectively. The radius of the SWGS microring refers to the distance between the center of the outermost ring waveguide and the center of the ring.

[0037] In a further embodiment, the sub-wavelength grating waveguide 9 of the SWGS straight waveguide and the SWGS micro-ring has the same number of silicon pillars in a one-to-one correspondence. The sub-wavelength grating period satisfies λ / Λ>>2n eff , λ is the incident wavelength, Λ is the period length of the sub-wavelength grating, n eff is the effective refractive index of the waveguide.

[0038] In a further embodiment, the grating period of the inner wall of the innermost ring SWG waveguide of the micro-ring group is 220nm, the grating period of the SWGS straight waveguide is 250nm, the waveguide width is 400nm, and the thickness of the silicon waveguide layer is 220nm.

[0039] In a further embodiment, the coupling spacing between the microring group and the upper and lower SWGS straight waveguides and the coupling spacing between the upper and lower SWGS microrings in the microring group are the same, both of which are 100 nm.

[0040] In a further embodiment, the gap spacing between the SWGS straight waveguide and the SWGS micro-ring is the same, which is 100 nm.

[0041] In a further embodiment, the transition waveguide structure is formed by an MMI converter 10, and the MMI converter includes two parts: a symmetrical 1×2 MMI and a tapered region. The length L of the MMI region ismmi and width W mmi They are 2 μm and 1.6 μm respectively, and the total length L of the converter is 26 μm.

[0042] In a further embodiment, the low refractive index upper cladding material is air.

[0043] In this embodiment, the vernier effect can be used to convert small changes in the external environment into the offset of the central wavelength of the resonance peak. The sensor with the SWGS double-ring resonance structure coupled in series is optimized, and the sensor with the SWGS triple-ring resonance structure coupled in series is further designed and optimized. Compared with the traditional strip waveguide, the light is not only distributed on the top and side of the sub-wavelength grating waveguide, but also distributed between the silicon pillars on the propagation path, so that the overlap between the light and the substance to be measured is increased. Therefore, the sub-wavelength grating multi-groove waveguide micro-ring resonance structure can effectively increase the contact between the analyte and the light, which is more conducive to improving the sensitivity of the device.

[0044] In this embodiment, the broadband optical signal is input into the resonator from the left end of the upper SWGS straight waveguide through the input strip waveguide-subwavelength grating groove waveguide transition waveguide structure, the resonant wavelength is coupled into the upper SWGS microring, and the non-resonant wavelength is output from the right end of the first straight waveguide 7. If the optical signal in the upper SWGS microring meets the resonance condition of the lower SWGS microring at the same time, it will be coupled into the lower SWGS microring and finally output from the right end of the lower SWGS straight waveguide. At this time, the received signal is the transmission spectrum when the cladding is air (refractive index is 1), which serves as the reference information for sensing. Then, the sample to be tested is added to the sensing structure. The sample to be tested will change the effective refractive index of the SWGS ring waveguide through the evanescent field, thereby changing the output spectrum of the entire sensor. Therefore, by detecting the output spectrum of the right end of the upper SWGS straight waveguide and comparing it with the calibrated reference information, the refractive index parameter of the sample to be tested can be obtained.

[0045] Figure 3 The transmission spectra of the SWGS double ring resonant structures with radii of 5μm and 7.35μm in pure water are shown in Figure 2. At this time, the refractive index of the analyte cladding is 1.3364. It can be seen from the figure that the resonance valleys of the two SWGS double rings are well aligned near the wavelengths of 1510nm and 1570nm. The FSRs of the two SWGS double rings are 21.39nm and 32.09nm, respectively, and the ratio is 2:3.0005, which is close to the ratio of the radii of the two SWGS double rings, that is, R 5μm : R 7.35μm =FSR 5μm :FSR 7.35μm .

[0046] Figure 4The transmission spectrum of the sensor in pure water is shown in Figure 1. The FSR of the sensor with two SWGS dual-ring resonant structures coupled in series is 62.59 nm, which is approximately equal to three times the FSR of the SWGS dual-ring resonant structure with a radius of 5 μm and twice the FSR of the SWGS dual-ring resonant structure with a radius of 7.35 μm. 5μm、7.35μm =3FSR 5μm =2FSR 7.35μm According to the calculation formula of Q factor: Q = λ res / Δλ FWHM , where Δλ FWHM It is expressed as the difference in wavelength corresponding to half of the resonance peak value, also called half-height width, λ res It can be calculated that the Q of the sensor with two SWGS double-ring resonant structures coupled in series is 585.4.

[0047] Figure 5 The curve of the resonant wavelength of the sensor with two SWGS double-ring resonant structures with radii of 5μm and 7.35μm coupled in series and the change of refractive index. It can be seen from the figure that as the refractive index increases, the transmission spectrum shifts to the right. According to the sensitivity calculation formula: S c =Δλ res / Δn c , where Δλ res Indicates the shift of the center wavelength of the resonance peak, Δn c The S of the sensor with two SWGS dual-ring resonant structures coupled in series can be calculated. c =540nm / RIU, and the refractive index sensing range reaches 0.116RIU within the FSR of 62.59nm.

[0048] Figure 6 The light field transmission distribution of the sensor with two SWGS double-ring resonant structures with radii of 5μm and 7.35μm in series when the resonant wavelength is 1.57μm. It can be seen from the figure that the TE mode can be smoothly output from the upper SWGS straight waveguide and the right end of the lower SWGS straight waveguide.

[0049] Figure 7 The planar structure diagram of the sensor is a series coupling of two SWGS three-ring resonant structures with radii of 6.6μm and 9.9μm. A SWG micro-ring is added to the SWGS single groove (i.e. double silicon pillar ring) to form a SWGS double groove (i.e. three silicon pillar ring) structure, which further improves the Q factor and sensitivity of the device.

[0050] Figure 8The transmission spectrum of the sensor in pure water is two SWGS three-ring resonant structures with radii of 6.6 μm and 9.9 μm coupled in series. The half-width is 1.49 nm, and the central wavelength of the resonance peak is 1584.89 nm. According to the Q factor calculation formula, the Q of the sensor coupled in series with two SWGS three-ring resonant structures can be calculated to be 1.23×10 3 .

[0051] Fig. 9 The curve of the resonant wavelength of the sensor coupled in series with two SWGS three-ring resonant structures with radii of 6.6μm and 9.9μm varies with the refractive index. When the refractive index changes from 1.3364 to 1.3464, the center wavelength of the resonance peak shifts to the right by 5.61nm. According to the sensitivity calculation formula, the S of the sensor coupled in series with two SWGS three-ring resonant structures can be calculated. c =561nm / RIU.

[0052] The purpose of the present invention is to provide an on-chip refractive index sensor based on a serially coupled sub-wavelength grating slot (SWGS) waveguide micro-ring resonant structure. Two sensors with serially coupled SWGS double-ring and triple-ring resonant structures are designed respectively, and the serial coupling between micro-rings and the vernier effect are utilized to set micro-rings of different radii, thereby increasing the free spectrum range and the measurable refractive index range of the material to be measured, and by adopting a sub-wavelength grating slot waveguide, a stronger light confinement and lower light dissipation structure are provided in the low-refractive index slit region, and when the sub-wavelength grating slot waveguide is exposed to the external environment, it is completely covered and filled by the detection material, which will enhance the interaction between light and the external environment, thereby improving the sensing sensitivity of the device.

[0053] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

[0054] It should be understood that in order to simplify the present invention and help those skilled in the art understand the various aspects of the present invention, in the above description of the exemplary embodiments of the present invention, the various features of the present invention are sometimes described in a single embodiment or described with reference to a single figure. However, the present invention should not be interpreted as the features included in the exemplary embodiments are all necessary technical features of the patent claims.

[0055] It should be understood that the modules, units, components, etc. included in the device of an embodiment of the present invention can be adaptively changed to be set in a device different from the embodiment. The different modules, units, or components included in the device of the embodiment can be combined into one module, unit, or component, or they can be divided into multiple sub-modules, sub-units, or sub-components.

Claims

1. An on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure, characterized in that: The invention comprises a silicon-based substrate (1), a buried oxide layer (2), a silicon waveguide layer (3) and an upper cladding layer (4), wherein the buried oxide layer (2) is grown on a first surface of the silicon-based substrate (1), the silicon waveguide layer (3) is grown on a side of the buried oxide layer (2) away from the silicon-based substrate (1), and the upper cladding layer (4) covers a side of the buried oxide layer (2) away from the silicon-based substrate (1) and wraps the silicon waveguide layer (3).

2. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 1, characterized in that: The silicon waveguide layer (3) comprises a micro-ring group, a first straight waveguide (7) and a second straight waveguide (8); the first straight waveguide (7) and the second straight waveguide (8) are arranged in parallel; the micro-ring group is arranged between the first straight waveguide (7) and the second straight waveguide (8); two ends of the first straight waveguide (7) are respectively connected with transition structures for realizing the transition between the input strip waveguide and the first straight waveguide (7) or the second straight waveguide (8); the transition structure is mirror-symmetrical with respect to the micro-ring group, the first straight waveguide (7) and the second straight waveguide (8) in the middle.

3. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 2, characterized in that: The microring group includes at least two SWGS microrings.

4. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 3, characterized in that: The radius of each SWGS microring in the microring group is not exactly the same.

5. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 3, characterized in that: Each SWGS micro-ring contains two SWG micro-rings with different radii, and the two SWG micro-rings form a concentric ring structure.

6. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 3, characterized in that: The micro-ring group, the first straight waveguide (7) and the second straight waveguide (8) are all sub-wavelength grating groove structures.

7. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 6, characterized in that: The first straight waveguide (7), the second straight waveguide (8) and each SWGS microring in the microring group have the same number of silicon pillars.

8. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 6, characterized in that: The sub-wavelength grating period of the first straight waveguide (7), the second straight waveguide (8) and the micro-ring group satisfies λ / Λ>>2n eff , λ is the incident wavelength, Λ is the period length of the sub-wavelength grating, n eff is the effective refractive index of the waveguide.

9. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 6, characterized in that: The coupling spacing between the micro-ring group and the first straight waveguide (7) and the second straight waveguide (8), the coupling spacing between the SWGS micro-rings in the micro-ring group, and the gap spacing between the first straight waveguide (7), the second straight waveguide (8) and the SWGS micro-rings are less than 300 nm.

10. The on-chip refractive index sensor based on a serially coupled sub-wavelength grating groove waveguide micro-ring resonant structure according to claim 1, characterized in that: The material of the upper cladding layer (4) is air.

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